1. Field of the Invention
This invention relates to the reactor vessel internals of a nuclear reactor, and more particularly to the means of affixing the component parts of the internals. It provides preferred fasteners and an arrangement to cool the fasteners which experience high operating mechanical loads and temperatures, by utilization of heat transfer to the flowing reactor coolant fluid.
2. Description of the Prior Art
The functions of the reactor internals of a nuclear reactor include support of the core components, such as the fuel assemblies, guidance of reactor coolant flow, and support of core monitoring apparatus. Most of the supported load is transmitted through the wall of the massive core barrel which surrounds the reactor core. Between the core barrel and the core is typically a baffle-and-former arrangement, also supported by the core barrel, which guides coolant flow through the core and provides an annulus that shields the core barrel wall from excessive irradiation. Typically, the baffle plates closely surround the core outer periphery so as to minimize bypass flow, which bypass flow would otherwise lower the thermal efficiency of the reactor. Because of the baffle plates proximity to the core, they experience a high temperature or high heat generation load. This results in a thermal gradient and high mechanical loads in the baffles, formers, core barrel, and the fastening means affixing these components.
Also, the barrel, which must transmit the massive load of the lower internals and fuel assemblies, is a thick structure, typically greater than 2 inches across the barrel wall. The baffle, which is not a similar load supporting structure, is thinner, typically about one inch thick. The formers which support the baffle are typically about 1.4 inch thick, and are affixed to both the baffle, which experiences a relatively high fluid temperature, and the barrel which experiences a relatively low fluid temperature. The actual temperature of these components may also vary during a fuel operating cycle such that the highest temperature component is different at different times in the operating life. The formers and the means which affix the formers to the other components, are therefore subject to a high thermal gradient. Further, the thermal expansion along the length of the relatively thick barrel compared to the thermal expansion of the relatively thin baffle results in a differential expansion that must be accommodated by the means which fasten these components. Over the 30 or 40 year design life of a nuclear plant, the thermal gradients and high mechanical loadings can weaken the fasteners potentially resulting in failure.
The means affixing the formers to the barrel and baffle have typically included a plurality of welds, or, more typically fasteners, such as bolts which are subject to high surface temperatures, which temperatures may rise above the saturation temperature of the coolant at its operating pressure. Some of these bolts penetrate the barrel wall from the outer periphery, and other bolts penetrate the baffle from the interior. From the foregoing discussion, it can be seen that these fasteners are subject to a high thermal load, and are also subject to thermally induced mechanical cyclic stresses. Because of the high mechanical loading, the body or shank of the fasteners is typically surrounded by an annulus within one of the components, such as the former. Early designs of fasteners were rather short bolts with relatively large shank diameters to minimize the thermal concerns. However, due to the large mechanical loadings, longer and thinner fasteners are now typically used. This, however, compounds the thermal concerns, as the fastener annulus is a barrier to heat transfer, resulting in additional heat loading of the fastener. Also, failure of such bolts has typically occurred at the junction of the head to the shank, where a rather abrupt cross-section transition occurs. Further, if coolant leaks into the annulus, it may boil due to the high fastener surface temperature in the annulus, thereby allowing chemical deposits to build up. The deposits will further decrease the heat transfer from the fasteners, resulting in even greater fastener temperatures, and greater potential for failure.
It is, therefore, desirable to provide a fastener and arrangement which will overcome these concerns of the prior art. Such an arrangement should continuously cool the fasteners during operation, so as to minimize induced thermal stresses. It should not allow boiling about the fastener surface. It further should not require excessive amounts of coolant bypass flow which would detract from the overall reactor thermal efficiency. And, it should not require significant modifications in the barrel, baffle, or formers, or add significant processes to the manufacturing of those components.